1. Field of the Invention
The present invention relates to a method of evaluating residual life of heat resistant steels which are used under stressed conditions at high temperatures.
2. Description of the Prior Art
For instance, superheater tubes of boilers in power stations exhibit deterioration of the material during long use becaue they are used under severe conditions at high temperatures and pressures. If the life of the material is completely consumed, accident such as rupture may occur. In order to avoid such accident, various methods have been known for evaluating residual life of heat-resistant steels such as a creep rupture test of the material, a method relying upon examination of change in the mechanical strength such as hardness or change in structure or a method which employs non-destructive examination for detecting any crack.
Destructive examination such as a creep rupture test, however, requires laborious works such as sampling by cutting of actually used parts and long-term examination which is to be conducted for a long time well exceeding several thousands of hours. Thus, it is impossible to evaluate residual life in a short time and in a non-destructive manner with such a method. The method relying upon detection of change in the mechanical properties such as hardness could not provide accurate information concerning the residual life over a long period from the beginning until the end of use partly because the change in the mechanical properties takes place mainly in the end portion of the life and partly because the changing characteristics vary largely. It has been also known that the change in the structure during long use is one of the most critical factors of the deterioration of the material. However, no effective method has been proposed for enabling quantitative analysis of the change in the structure of heat-resistant steels for the purpose of evaluation of residual life. Non-destructive examination for detecting cracks in heat-resistant steel used at high temperature and stress is very effective for the purpose of evaluation of residual life but this method cannot provide any information concerning the state of material before the crack is actually generated.
The present invention has been developed to fill the gap between the level of the conventional art and the actual demand, by providing a method which enables an accurate evaluation of residual life of heat-resistant steel from information concerning a change in the density etc. of alloy element in heat-resistant steel.
To this end, according to the present invention, there is provided a method of evaluating the residual life of heat-resistant steel used at high temperature and pressure, having the following features.
According to a first aspect, there is provided a method for evaluating residual life of a heat-resistant steel after a service at high levels of temperature and stress comprising: measuring the density of an alloy element in a tiny region of each of a multiplicity of measuring spots on the surface of the heat-resistant steel; and obtaining a prediction value of the residual life by locating the data concerning the measured values of density of the alloy element on a residual life evaluation reference diagram which has been beforehand obtained and which represents the relationship between the density of the alloy element and the life consumption as obtained through a non-destructive test or a stress analysis on the heat-resistant steel used under various conditions.
According to a second aspect of the invention, there is provided a method for evaluating the residual life of a heat-resistant steel after a service at high levels of temperature and stress, comprising: obtaining a replica from the surface of the heat-resistant steel; determining the state of deterioration of metallurgical structure of the heat-resistant steel and/or the state of precipitates from the replica; and obtaining a prediction value of the residual life by locating the data concerning the determined values the state of deterioration of metallurgical structure and/or the state of precipitates on a residual life evaluation reference diagram which has been beforehand obtained and which represents the relationship between the state of deterioration of metallurgical structure and/or the state of precipitates and the life consumption as obtained through a non-destructive test or a stress analysis on the heat-resistant steel used under various conditions.
The present inventors have found, through an intense study concerning heat-resistant steel used at high temperature and pressure, that a very close correlation exists between the residual life of heat-resistant steel and state of densities of alloy elements of the steel and state of degradation of metallurgical structure of the steel and/or state of precipitation.
The present invention is based upon such a discovery. According to the invention, densities of alloy elements in tiny areas on the surface of the heat-resistant steel used is measured at a multiplicity of points or, alternatively, the state of degradation of metallurgical structure and/or state of precipitation is determined from a replica extracted from the surface of the heat-resistant steel. At the same time, correlations between these factors and the life consumption rate are beforehand obtained through a destructive test or stress analysis and data thus obtained are stored in the form of a residual life evaluation reference diagram. The residual life of the heat-resistant steel, therefore, can be evaluated by making reference to such a diagram.
According to the present invention, therefore, it is possible to accurately evaluate in a short time without necessitating any destructive exmination. In addition, the method of the invention can provide information concerning the residual life of the heat-resistant steel over a long period of time from the beginning of the use until the end of the service.